A four-month-old truce frayed Thursday as Palestinian militants fired mortars and rockets at Jewish targets for a second straight day and Israeli security chiefs threatened retaliation.

Senior Palestinian and Israeli officials met to try to defuse the escalating violence and protect a cease-fire agreement seen as a key to renewing peace negotiations.

Dozens of projectiles fell on Jewish settlements in the Gaza Strip (search), and a mortar shell exploded at an Israeli army base just outside Gaza, causing some damage but no injuries. Two rockets hit the Karni crossing point into Israel, the military said. After nightfall, a rocket landed in the yard of a house in Sderot, an Israeli town just outside Gaza, but did not explode.

Defense Minister Shaul Mofaz (search) called military commanders in for consultations, and participants reported that he warned Israel could resume targeting anyone seen about to fire mortars — as it did Wednesday, killing a Hamas member.

Such raids would likely trigger further violence by Hamas (search), which claims the right to retaliate for Israeli strikes, and a tit-for-tat escalation could shred the truce that has significantly reduced bloodshed since Feb. 8.

Still, there were undercurrents indicating reluctance to return to the combat of the preceding four years, when more than 4,000 people died from Palestinian suicide bombings in Israel, Israeli airstrikes and ground operations in Gaza, Palestinian roadside ambushes of civilians in the West Bank and Israeli assassinations of suspected militants.

Hamas leader Mahmoud Zahar said the cease-fire remained in effect, though "we are committed to defend ourselves in the face of any (Israeli) aggression in the West Bank and Gaza."

The Palestinian Interior Ministry issued a strong criticism of Hamas after civilians and Hamas loyalists chased off Palestinian police officers who tried to deploy Wednesday in Khan Younis, a Gaza refugee camp that is the source of most of the mortar and rocket fire. But there was no noticeable effort to deploy police Thursday.

Israeli Deputy Defense Minister Zeev Boim told Israel TV the violence is tied to this summer's Palestinian parliamentary election, in which Hamas is fielding candidates for the first time and is expected to do well against Fatah, the party of Palestinian leader Mahmoud Abbas.

"We are not prepared to pay the price for that," Boim said.

However, he tempered his usual tough style by crediting Abbas with trying to restore order. "We want to support the calm and the efforts of Abu Mazen," Boim said, referring to Abbas by his nickname. "He has not shown many results, but his intentions are good."

In another signal, officials said Mofaz planned to give the Cabinet within 10 days a list of 400 Palestinian prisoners to be freed. Israel pledged to release prisoners as part of the truce but stopped the process, accusing the Palestinians of not holding up their end of the deal.

Israeli officials warned a resurgence of violence could unhinge Israel's planned withdrawal from Gaza and part of the West Bank this summer, which would be a blow to both sides.

Israeli Prime Minister Ariel Sharon has all his political capital invested in pushing ahead with the pullout in the face of fierce domestic opposition, while Abbas could claim the removal of settlements as an accomplishment for his administration.

Also, Abbas is scheduled to travel to Washington next week for talks with President Bush, and violence could cloud the visit or even lead to its cancellation.

Abbas, meanwhile, rang up two more small victories in internal politics Thursday when Gaza courts ordered partial recounts in May 5 local elections in two locations where Hamas swept to victory — the town of Beit Lahiya and the Bureij refugee camp.

Thousands of Hamas supporters in both places later marched through the streets in protest, calling on the courts to let the original results stand. Smaller numbers of Fatah backers were out celebrating, but there were no confrontations between the sides.

Earlier, a court ordered a partial revote in Rafah in the local election, which was seen as a harbinger for parliamentary balloting.

Hamas expects to tap into voter disaffection with 10 years of Fatah rule laced with corruption, inefficiency and nepotism, anger that has surfaced since the death of leader Yasser Arafat in November.

Abbas, his successor, has been trying to institute reforms, but he lacks Arafat's mythic aura.